1. Field of the Invention
The present invention relates to a fiber Bragg grating and, particularly, to a dynamic fiber Bragg grating whose state of reflection can be tuned over a continuous range.
2. Related Art
Conventionally, a fiber Bragg grating (FBG) is formed by exposing an optical fiber in a periodic UV light pattern generated either by two UV beams interfering or one UV beam passing through a phase mask. The materials composing the optical fiber core are photosensitive and absorb UV photons. When the materials absorb photons, the index of refraction (n) of the core increase. Thus, after a process of exposure, a periodic pattern will be present in the optical fiber core, regions that have absorbed UV light will have an increased index of refraction, and regions that have not absorbed UV light will have an unchanged index of refraction. When an incident light travels through the FBG, the wavelength, which meets the Bragg condition (xcex=2neffxcex9, where neff is the effective index of refraction of the optical fiber core and xcex9 is a pitch of the FBG which is the distance between two UV exposed areas), is reflected back (or rejected) by the FBG The reflection ratio of the FBG is determined by the length of the FBG and a difference in indexes refraction of the UV exposed and the non-exposed areas.
However, because the resulting pattern formed by exposure in the optical fiber core is permanent, the properties of the FBG, especially, the state of reflection of the FBG, is fixed by the UV exposure process and can not be tuned. Thus, this kind of FBG is static. Moreover, this kind of FBG requires optical fiber materials which are photosensitive, which essentially limits such FBGS to be made from silica-based materials.
Accordingly, an improved fiber Bragg grating is desired to overcome the above problems.
An object of the present invention is to provide a dynamic fiber Bragg grating (FBG) whose state of reflection can be tuned over a continuous range.
Another object of the present invention is to provide an FBG which can be based on a wide variety of material.
To achieve the above object, an FBG in accordance with a preferred embodiment of the present invention includes an optical fiber having an optical fiber core made of a material whose index of refraction varies with temperature, a thermal electric cooler mounted on the optical fiber for maintaining xe2x80x9ccold spotsxe2x80x9d, and a resistor including a plurality of micro heating elements arranged on the optical fiber for forming periodic xe2x80x9chot spotsxe2x80x9d. A temperature of the xe2x80x9chot spotsxe2x80x9d varies with a current flowing through the resistor, thereby controlling an index of refraction of narrow cross sections of the optical fiber core. Therefore, a state of reflection of the fiber Bragg grating can be tuned over a continuous range.